RFC proposal: fsync support for file resources

No, fflush will flush the PHP application buffers out to the operating
system, it does in contrast to fsync not provide a guarantee the operating
system will immediately persist to the underlying storage device.

Hello David,

isn't fflush exactly this:
https://www.php.net/manual/en/function.fflush.php

Greets
Dennis

Am 01.06.20 um 18:56 schrieb David Gebler:

Exactly as the subject says, I would like to propose an RFC for adding an
fsync() function for file resources, which would in essence be a thin
wrapper around C's fsync on UNIX systems and _commit on Windows.

It seems to me an odd oversight that this has never been implemented in
PHP
and means PHP has no way to perform durable file write operations, making
it inherently unsuitable for any systems requiring more intensive I/O,
mission critical logs, auditing, etc.

I am not really a C programmer and I have been able to implement a simple
working prototype of this as a compiled extension in merely a few hours,
so
I'm sure it wouldn't be difficult to bring in to the language core where
the functionality really belongs.

Every other major programming language otherwise comparable to PHP in
features supports a way of providing durability.

Thanks.

fsync() does not guarantee data has been persisted because no software
mechanism which exists can do that. It is however the best assurance we
can have that data has been physically persisted by the time the function
returns.

fsync() is the best operating-system level assurance we can get regarding
data persistence and PHP is the only major programming language I can think
of which doesn't support a means of doing it. I've had use cases in web
systems and microservices written in PHP where the ability to provide
durable file writes would have been very valuable, other cases where
services have had to be written in Java or some other language specifically
because they require durability where PHP might otherwise have been a good
candidate.

While there is little reason not to add fsync() to PHP the importance and
benefits are IMHO not that big.

Arguably so, but neither that big is the effort required to implement this
feature.

On Mon, Jun 1, 2020 at 8:47 PM Christian Schneider cschneid@cschneid.com
wrote:

Am 01.06.2020 um 18:56 schrieb David Gebler davidgebler@gmail.com:

It seems to me an odd oversight that this has never been implemented in
PHP
and means PHP has no way to perform durable file write operations, making
it inherently unsuitable for any systems requiring more intensive I/O,
mission critical logs, auditing, etc.

I think there is a misconception of the usefulness of fsync() here.

First of all it does not really guarantee the data is safe from power
outages etc, see
https://stackoverflow.com/a/706688
https://stackoverflow.com/a/725742
and other answers in that thread.

Secondly I'd rather rely on something like a proper DB if not losing any
data is crucial.
Trying to implement it in pure PHP and plain old files seems like using
the wrong tools for the job to me.

While there is little reason not to add fsync() to PHP the importance and
benefits are IMHO not that big.

• Chris

It's interesting that you mention fdatasync - perhaps a better C programmer
than I am can correct me, but I don't think it has an equivalent on
Windows, where _commit() is a wrap on FlushFileBuffers API. So in respect
of PHP implementation, the options would be:

fdatasync() is not available on Windows, but fsync() is. fdatasync() is
available on UNIX builds.

fdatasync() on Windows is an alias of fsync(), on UNIX fdatasync() is as
expected.

Only fsync() is implemented for both Windows and Unix.

-Dave

Exactly as the subject says, I would like to propose an RFC for adding an
fsync() function for file resources, which would in essence be a thin
wrapper around C's fsync on UNIX systems and _commit on Windows.

It seems to me an odd oversight that this has never been implemented in
PHP
and means PHP has no way to perform durable file write operations, making
it inherently unsuitable for any systems requiring more intensive I/O,
mission critical logs, auditing, etc.

I am not really a C programmer and I have been able to implement a simple
working prototype of this as a compiled extension in merely a few hours,
so
I'm sure it wouldn't be difficult to bring in to the language core where
the functionality really belongs.

Every other major programming language otherwise comparable to PHP in
features supports a way of providing durability.

Thanks.

No objections from my side. I assume you'd want to add both fsync() and
fdatasync()?

I think all it takes for this one is a PR...

Regards,
Nikita